Update alpine Docker tag to v3.21 (main)

[red-hat-konflux]

This PR contains the following updates:

Package	Type	Update	Change
alpine	final	minor	3.18 -> 3.21

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This PR has been generated by Renovate Bot.

[openshift-ci]

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
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Measures
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0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[openshift-ci]

@red-hat-konflux[bot]: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/test-e2e	91591be	link	true	/test test-e2e

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